Method and system for synchronizing base station and establishing location

ABSTRACT

With the increasing usage of mobile devices for communication, the need for wireless base-stations deployed in strategic locations is becoming increasingly important. The increased bandwidths being transmitted between the base-station and the mobile device has mandated that enhanced transmission formats and techniques be deployed, and, in order to operate correctly, these techniques require a tight synchronization in both time/phase, and in frequency, between the various base-stations serving a general area. Due to the need to establish the geographic location of the mobile device with a high degree of accuracy, it is also necessary to establish the location of the serving base-stations with a high degree of accuracy. The invention disclosed herein provides robust and practical methods for synchronizing base-stations, as well as providing for accurate location, by leveraging the usage of global navigation satellite systems receivers in conjunction with network based schemes for packet-based (time/phase/frequency) synchronization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application havingSer. No. 61/920,176, filed on Dec. 23, 2013, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Description of the Related Art

With the increasing usage of mobile devices for communication, the needfor wireless base-stations deployed in strategic locations is becomingincreasingly important. Such base-stations are often referred to as“small cells” since they cover a limited geographic area and are often,but not always, paired with a “macro cell” that involves a base-stationwith much larger geographic coverage. The increased bandwidth beingtransmitted between the base-station and the mobile device has mandatedthat enhanced transmission formats and techniques be deployed, and, inorder to operate correctly, these techniques require a tightsynchronization in both time/phase, and frequency, between the variousbase-stations serving a general area.

The need for providing emergency services (also referred to in the USAas E-911) has mandated that the geographic location of the mobile devicesignaling an emergency be established to a high degree of accuracy. Inorder to universally achieve this location accuracy, it is necessary toestablish the location of the serving base-stations to a high degree ofaccuracy.

The use of GNSS for providing location and time at a receiver is known.The most common GNSS in use is the Global Positioning Satellite (GPS)system. The use of navigational satellites to establish the position andtime in a receiver is briefly explained with reference to panel A ofFIG. 1. For specificity, the satellite constellation considered is GPS.

As shown in FIG. 1, the receiver (REC) 110 can establish its positionand time by observing 4 satellite vehicles (SVs) denoted in FIG. 1 asSV-1 121, SV-2 122, SV-3 123, and SV-4 124. Each SV is broadcasting asignal that includes suitable time markers and messages indicating thetime at the marker. At any particular point in time, t, the foursatellite vehicles are at a particular distance (range) from thereceiver 110. In FIG. 1 the actual range of the four satellite vehiclesis denoted by R₁(t) 131, R₂(t) 132, R₃(t) 133, and R₄(t) 134. The rangecan be expressed either in terms of distance (e.g., meters) or in termsof time (e.g., seconds). Here we assume that the range is in terms oftime because a time formulation permits the inclusion of correctionterms related to effects of the ionosphere. The relationship betweentime and distance follows the speed of propagation of the radio signal,essentially the speed of light (nominally 3×10⁸ m/s).

Denote the time offset of the receiver clock as δ. The receiverestimates the range from SV-k by establishing the time-of-arrival of thetime marker event contained in the broadcast signal from SV-k. Thistime-of-arrival is based on the receiver clock. The range estimate,PR_(k), is called “pseudo-range” because of the receiver time offset.Thus at some point in time, T, the receiver has four range estimates,{PR_(k)(T); k=1,2,3,4}. Since, in principle, the location in space ofthe four satellite vehicles is known, the receiver can develop a set offour simultaneous equations and solve for the four unknownscorresponding to position (x_(R), y_(R), z_(R)) of the receiver and δ,the receiver clock offset (relative to the satellite timescale).

Packet-Based Methods for Time/Phase/Frequency Synchronization

The distribution of time over packet networks is now ubiquitous. Thedominant method is the use of Network Timing Protocol (NTP) for supportof general timing applications in general computing applications.However, these implementations, based on existing standards andconventions, are suitable for time alignments on the order of (several)milliseconds. Over the last decade, a new protocol, Precision TimingProtocol (PTP) has emerged supported by industry standards (IEEE 1588v2,ITU G.827x series). The key differentiator between NTP and PTP is thelevels of precision obtainable with PTP can support the needs of avariety of new applications and services. Both PTP and NTP are protocolsfor exchanging time-stamps associated with time-of-arrival andtime-of-departure of designated packets and are thus, in principle ifnot practice, capable of similar performance levels.

Combined Methods for Synchronization

U.S. Patent Application Publication 2014/0192826, entitled “UNIVERSALASYMMETRY CORRECTION FOR PACKET TIMING PROTOCOLS” provides methods forutilizing both GPS/GNSS methods and packet-based methods in acollaborative manner to provide a robust scheme for distributingtime/frequency between devices. The description of such techniques isprovided in U.S. Patent Application Publication 2014/0192826, which isincorporated by reference herein in its entirety.

In all network-based synchronization schemes the single, dominant,source of time error, error that cannot be corrected by the protocol, isasymmetry. Asymmetry as considered here is the difference in transitdelay of the designated event packets in the two directions between thecommunicating clocks. Whereas packet delay variation is an expectedphenomenon in packet networks and contributes to asymmetry, there is anunderlying asymmetry component that is entirely independent of thenetwork loading and depends substantively only on the path between theclocks. The path, as considered here, includes all transmission links,including multiplexers and signal-format converters and transmissionmedia, and intermediate network elements, such as switches and routers,between the communicating clocks.

One approach known in the art that provides both time and frequencyalignment involves computing an aligned time signal based on a mastertiming signal from a primary reference clock, such as a GNSS satellitetiming signal, which is held in precise alignment with a global clockreference. This is depicted in panel B of FIG. 1.

The two clocks, CLOCK-1 110 and CLOCK-2 120 both receive timing signals130 from the GNSS system 150. By aligning themselves to the GNSStimescale, the two clocks are, albeit indirectly, aligned to each other.Using GPS signals or other master timing signals at each network elementto achieve time or frequency alignment is generally prohibitivelyexpensive and requires each network element to be able to receivesatellite time signals from GPS satellites. There are many situationswhere visibility of GPS satellites may be compromised, interfered with,or interrupted. It is generally accepted that, whereas GPS is a reliableand robust system, in many installations the visibility of a sufficientnumber of satellites simultaneously may not be available continuously.

As described in U.S. Patent Application Publication 2014/0192826, thetwo clocks 110 and 120 can ascertain the network asymmetry between themby first aligning themselves to the common GNSS timescale. This enablesthe clocks to synchronize themselves when the GNSS signal iscompromised, interfered with, or interrupted.

SUMMARY OF INVENTION

Embodiments disclosed provide robust and practical methods forsynchronizing base-stations (“small cells” and “macro cells”) as well asproviding for accurate location. Some embodiments leverage the usage ofglobal navigation satellite systems (GNSS) receivers in conjunction withnetwork based schemes for packet-based (time/phase/frequency)synchronization.

We assume that the receiver is (nominally) fixed in position. Byrepeatedly developing range estimates and solving forlocation/time-offset, the receiver can mitigate the impact of noiseassociated with (range) measurement and clock noise in the receiverclock subsystem.

Very often the receiver has multiple, possibly more than four,satellites in view and can develop pseudo-range estimates to more than 4satellites. This results in more than 4 equations and this permits theapplication of mathematical techniques for noise mitigation.

This disclosure describes several additional methods for developingmultiple equations, the premise being that having an “over-determined”set of equations (more equations than unknowns) enables the use of knownmathematical tools for minimizing the impact of measurement and/or clockerrors.

One of the enabling items is that the different receivers/clocks can beoperated in a collaborative manner and thereby certain measurements thathave significant error can be detected and not used in the solution.

Another one of the enabling items is that the different clocks cansyntonize (i.e. synchronize in terms of frequency) themselves and thushave a common frequency-base. This syntonization is achieved usingpacket-based methods over the network. This permits the group of clocksto be syntonized (frequency alignment) using packet-based methods (PTP)and synchronized (time alignment) using GNSS or PTP.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 depicts the notion of a receiver obtaining its time and frequencyfrom the GNSS system and the notion of two devices synchronizing witheach other by simultaneously obtaining time and frequency from the samenavigational satellite system.

FIG. 2 depicts how, depending on the mounting location of the GNSSantenna, part of the sky may be obscured.

FIG. 3 depicts the urban canyon effect wherein part of the sky may beobscured and satellites in some parts of the sky may be visible via anon-line-of-sight or multipath signal.

FIG. 4 depicts the case of multipath associated with a particularsatellite vehicle as it traverses the sky.

FIG. 5 depicts a situation where multipath in the case of two receiversin relatively close proximity can be detected by the receivers operatingcollaboratively.

FIG. 6 provides a simplified block diagram depicting the generation ofthe GPS L1 signal.

FIG. 7 provides a simplified block diagram of the GPS correlationreceiver.

FIG. 8 illustrates the correlation pattern computed via the correlationblock.

FIG. 9 depicts the situation of a collection of clocks with GNSS/GPSReceivers with one Master clock in the set.

FIG. 10 identifies the case where two clock devices with GNSS/GPSreceivers maintain a packet-based timing signal communication that isaffected by asymmetry as well as a communication channel to passparameters related to timing, location, and GNSS receiver signals.

FIG. 11 depicts a method for iteratively estimating the location/time ofGNSS receivers.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION The Urban Canyon Effect

One of the important considerations in deploying small cells with GPSreceivers is the “urban canyon effect”. In an ideal situation, thereceiver would have a good view of the sky and thereby have a directline-of-sight path to the satellites. In some cases the receiver antennais not mounted with a clear view of the sky. For example, the antennamay be mounted on the side of a building and the building may thusappear as an obstruction. This is depicted in FIG. 2.

As shown in FIG. 2, the receiver 110 may be on the side of a building210 that could obstruct the view of some satellites. In FIG. 2, thebuilding 210 obstructs the view of satellites SV-3 and SV-4 at time t.This obstruction is indicated by the range vectors R₃(t) 233 and R₄(t)234 being depicted with dashed lines. As shown, the receiver 110 canobserve the signals from SV-1 121 and SV-2 122. Consequently thereceiver 110 can make only 2 pseudo-range estimates. The two equationsdeveloped are insufficient to evaluate four unknowns, namely position(x,y,z) and time-offset (δ).

In practice, if the receiver antenna is fixed then the position may beknown and, further, can be assumed to be “constant”. If this is a validassumption then even though the receiver can observe just twosatellites, there is only one unknown, namely time-offset and this canbe estimated provided that there is at least one satellite in view. Whenthe primary utility of the receiver is to establish time and frequency,such obstruction can be considered relatively benign if the position ofthe receiver has been accurately established.

In the simple configuration of FIG. 2, exemplifying cases where thereceiver antenna is mounted on the side of a building, the receiver hasview of about one-half of the sky, assuming that the building depictedby its outline 210 is impermeable to the radio frequency (RF) signal.Depending on the orbit of a particular satellite vehicle, it is possiblethat the satellite is observable for much less of the time as comparedto the case where there is no obstruction.

In cases where the receiver antenna is mounted on the side of a buildingin an urban environment, there can be situations of multipath. This isdepicted in FIG. 3.

Of particular importance in FIG. 3 is the impact of obstruction 310 thatmay block the signal from the position of SV-1. It can result inreflections as well as blockage. In particular, FIG. 3 indicates thepath 333 that the signal from SV-3 can take to reach the receiver 110.Instead of being blocked from view, the receiver sees SV-3 over a paththat is longer than line-of-sight. This is the impact of multipath.Multipath is thus the phenomenon whereby signal from a satellite arrivesat the receiver but along a path that is other than line-of-sight andthereby provides an incorrect estimate of the true range, the estimatebeing greater than the estimate provided by the direct path.

Whereas the examples given above assume that the satellite constellationat a particular time instant is considered, a similar example can begenerated by considering a particular satellite vehicle, say SV-N, as ittraverses its orbit. There may be particular times when the satellite isclearly visible, other times when the satellite vehicle is blocked, andyet other times when the satellite signal arrives at the receiver via anon-line-of-sight path (multipath). An example of this situation isdepicted in FIG. 4.

As depicted in FIG. 4, one satellite, namely SV-N, is present at severaldifferent locations in the sky at different times. At time t1, satelliteSV-N is at location 421 and the range is R_(N)(t₁) 431, depicted as adashed line to point out that the satellite SV-N may be obscured byobstruction 310. From location 422, the range R_(N)(t₂) 432 isrepresentative of line-of-sight. From location 423, the range R_(N)(t₃)433 is not representative of line-of-sight transmission but is observedvia a reflected ray. At location 424, the signal is obscured byobstruction 210.

Of particular significance is the situation where there may be two GPSreceivers that are in relatively close geographical proximity. Thisprovides the opportunity to utilize the two receivers operating in acollaborative manner in order to mitigate the impact of multipath, whichis depicted in FIG. 5.

In FIG. 5, there are two receivers REC-1 511 and REC-2 512 that are bothreceiving a signal from SV-N that is at position 523 in the sky. Thepath to REC-2 512 is a line-of-sight path 543, whereas the path to REC-1511 is a non-line-of-sight path 533. The benefit of detecting themultipath nature is that REC-1 511 can account for that in its solutionfor time (and location).

In addition, FIG. 5 depicts another GPS receiver REC-M 550 located suchthat it is reasonably close to REC-1 511 (and REC-2 512) but has a clearview of the sky, particularly for satellite SV-N 523. This situation iscommon in base-station deployments wherein the “macro-cell” is generallythe large base-station that is deployed to support a wide geographicalarea and engineered such that it has an unobstructed view of the sky.For improving coverage and additional traffic, several “small cells” aredeployed in the general area but it may not be feasible to provide eachsmall cell unobstructed sky view. The small cells and macro-cellcollaborate to provide suitable wireless coverage and can alsocollaborate to improve location/timing performance as described herein.Generally the device with the best sky view is the Master clock of thecollection and the other devices function as Slave clocks.

Estimating Range

The GPS signal structure is based on direct sequence spread spectrummodulation. The principal features of the GPS transmit signal are shownin FIG. 6 by way of a simplified block diagram indicating the manner inwhich the GPS signal is constructed.

Each satellite vehicle has a designated spreading code from a collectionof Gold codes. The chip-rate used for the direct sequence spreading is1023 kHz so that there are exactly 1023 chips in a period of 1 ms. Thechosen Gold codes have a period of 1023 chips so that the period of theGold code in time is 1 ms. The information bit rate is 50 b/s so thatthere are 20 Gold code periods per information bit.

The clock generator 620 provides the clocking information to generatethe Gold code at the correct rate via the CHIP-CLK 622 and the 1 msperiod definition via CODE-SYNC 623 and the bit-clock BIT-CLK 624 to theinformation generator DATA GEN 640. The direct sequence spreading isachieved by the mixer 630. The composite (spread-spectrum) signal at thechip rate is applied to a BPSK (binary phase shift keying) modulator 650and the appropriate L1 carrier signal RF-CLK 625 to establish thecomposite RF signal 660. It should be noted that this is the C/A signaldefined in the GPS specifications.

A simplified block diagram of the receiver section that detects thepresence of the signal from the particular satellite vehicle andestimates the effective time delay and thereby the “pseudo-range” to theparticular satellite vehicle is depicted in FIG. 7. The basis of thereceiver is the correlation operation whereby the incoming signal isapplied to a “matched filter” that is matched to the particularspreading code of the particular satellite vehicle.

The antenna 710 receives the RF signal from the air and this isdown-converted to a suitable intermediate frequency (IF) signal. This IFsignal is converted into digital format using an analog-to-digitalconverter (ADC) to create a digital IF signal 725. The down-conversionand ADC is depicted as block 720.

The digital IF signal 725 is further demodulated to base-band usingdigital means with a Numerically Controlled Oscillator (NCO) 735. Thisapproach permits Doppler effect related to satellite motion to beaddressed. The demodulation is done in both in-phase and quadrature tocreate a complex baseband signal 736 that is fed to the complexcorrelator 750. The other leg of the correlator 750 is provided with thereference Gold code for this particular satellite vehicle beingsearched/tracked. The code generator 740 develops the chip patternagainst which the incoming RF signal, in the form of the complexbaseband signal 736, is compared. It is well known that the correlationoperation is associated with the matched filter, and this operationprovides the best signal-to-noise ratio for ascertaining whether thesatellite vehicle is being detected/tracked. The NCO 745 serves as thecode NCO and is adjusted to provide the code at the appropriate phasing.

Provided the signal from the satellite under observation is present, thecorrelator 750 may provide information such as that depicted in FIG. 8.The correlation pattern 820 is developed by the correlator 750 whichcomputes the correlation over 1 ms between the received signal 736 and alocal replica of the code developed by code generator 740 for differenttime lags. The correlation pattern 820 represents the magnitude of thecorrelation. The receiver estimates the time lag T_(peak) 840 by pickingthe peak value 825 of the correlation pattern. This time lag correspondsto the receivers estimate of the distance (range), expressed in timeunits, between the receiver and the satellite. Since there may be anoffset in the local receiver clock, this range estimate is referred toas “pseudorange”.

The pseudorange and true range are related by the equation:

PR(t)=R(t)+δ(t)  (Eq. 1)

where PR(t) is the pseudorange, R(t) is the actual range, and δ(t) isthe receiver clock offset. Depending on the convention used, the + signin the right-hand-side of (Eq. 1) may be changed to a − sign.

In practice, the estimate may be corrupted by multipath and, further andnoise in the correlation estimate. Consequently, the actual estimate ofpseudorange is related to the other terms as shown in (Eq. 2):

(t)=R(t)+δ(t)+m(t)+ε(t)  (Eq. 2)

In (Eq. 2) the pseudorange estimate is affected not just by the localclock offset, δ(t); but also the impact of multipath, m(t); and possibleerror introduced by noise that introduces an apparent shift of thecorrelation peak, ε(t). If the error introduced by multipath issignificant, then the location/time solution will have error in bothposition and time. Further, note that the multipath error is alwayspositive (or always negative depending on the prevailing convention) andtherefore conventional noise mitigation techniques such as averaging arenot very useful.

In some instances position is known to a reasonable accuracy and time isthe principal desired result from the calculation. Since, in principle,only one equation is required and additional equations serve to mitigatenoise (mainly the measurement noise ε(t)), it is advantageous toidentify the subset of pseudoranges that are substantively free ofmultipath error.

Receiver Coordination

Consider the case when multiple receivers, for example REC-1 511 andREC-M 550, can collaborate and exchange information regarding thepseudorange estimate to the same satellite from the multiple receiverlocations.

That is, using the subscripts 1 and M to represent the entities in thetwo receivers at time t=T₁, (Eq. 3A) represents the apparent pseudorangePR₁ and (Eq. 3B) provides the pseudorange for REC-M, PR_(M):

₁(t)=R ₁(t)+δ₁(t)+m ₁(t)+(t)  (Eq. 3A)

PR_(M)(t)=R _(M)(t)+δ_(M)(t)+ε_(M)(t)  (Eq. 3B)

Since REC-M has a clear view of the sky, there is no multipath termm_(M)(t).

Given that the two receivers are in reasonably close geographicproximity, the ranges R₁(t) and R_(M)(t) are related. Specifically, fora given general location on earth, the following limit can beestablished:

|R _(M)(t)−R ₁(t)|≦ρ_(M)  (Eq. 4)

The subscript M is used to refer to the notion that the range differenceρ_(M) is associated with the receiver that has the clear view of thesky, although it is not necessary for one or both of the receivers tohave a clear view of the sky for a relationship as expressed by (Eq. 4)to apply.

If it can be ensured that the time errors δ₁(t) and δ_(M)(t) are close,namely

|δ_(M)(t)−δ₁(t)<∈_(M)  (Eq. 5)

and the measurement errors ε₁(t) and ε_(M)(t) can be bounded as

|∈₁(t)|<ε_(MAX)

|∈_(M)(t)<ε_(MAX)  (Eq. 6)

then by examining the apparent pseudorange for REC-1 and the pseudorangefor REC-M, it can be concluded that if

|

₁(t)−PR_(M)(t)|>THRESHOLD_(HI)  (Eq. 7)

then the particular pseudorange measurement is excessively corrupted bymultipath. In one embodiment, the receiver may output a signalreflective of such multipath corruption. The value of THRESHOLD_(HI) isrelated to the known, pre-determined, limit ρ_(M) and the estimated timeerrors and estimated measurement errors. Furthermore, if

|

₁(t)−PR_(M)(t)|<THRESHOLD_(LO)  (Eq. 8)

then it is very likely that the particular pseudorange measurement isnot corrupted by multipath.

Timing Collaboration

In order to utilize receiver coordination, it is necessary to coordinatethe receiver clocks to a level close enough to reject multipath. As willbe shown, by coordinating the clocks of the various receivers, allreceiver clocks can be syntonized. This syntonization providesadditional benefits as will be explained later.

As depicted in FIG. 9, the GPS receiver with the clear (or at least thebest) view of the sky is configured as a PTP Master Clock 550 and theother GPS receivers REC-1, REC-2, and REC-3 are configured as slaveclocks 511-513. These elements are interconnected over a packet network.Given the geographic proximity of the receivers (PTP clocks), the numberof intervening switches between the clocks is likely to be small,typically 3 or less. PTP streams 910, 920, and 930 interconnect theslave clocks 511-513 to the master clock 550 and deliver timingreferences that allow the slaves to synchronize with the master.

Given that the master has a GPS receiver that has a clear view of thesky, the time-clock 550 is accurate. It is generally accepted that aclock with a good GPS receiver with a clear view of the sky can maintaintime with an accuracy of better than 100 ns. That is, |δ_(M)(t)|<100 ns.Given that the packet network interconnecting the clocks is small, thesyntonization that can be achieved between the slave clocks 511-513 andthe master clock 550 is excellent. That is, all the slave clocks 511-513may have frequency offsets that are essentially negligible. The timeoffset between the slave clocks 511-513 and the master clock 550 isbecause the asymmetry of transmission paths between the clocks isunknown.

Given that the slave clocks are syntonized with the master and, further,that the master is locked to GPS, the slave clock time offset, forexample of slave clock 511, δ₁(t), remains constant unless and until acorrection is applied.

The architecture is depicted in FIG. 10 showing one slave clock 511 withits associated GNSS (e.g., GPS) receiver. The intervening networkbetween the slave clock 511 and the master clock 550 may have multiplenetwork elements as well as multiple inter-machine transmission links.All these intervening items can introduce asymmetry.

The packet network 900 enables the establishment of a (logical)communication channel referred to as the “Optional Asymmetry MessagingChannel” 1065 that can be used to signal between the master and theslave and include information related to asymmetry as well asinformation regarding satellite information, particularly pseudo-rangemeasurements, as well as the current best approximation of thegeographical location. In the scenario being discussed the variousreceivers are assumed to be stationary and the location is, nominally, aconstant.

Enhanced Algorithm for GPS Solution

These observations form the basis of the following approach toestablishing and improving location estimates and time offset estimates.For purposes of explanation, the approach is described for receiverREC-1 511.

Denote the location and time offset of REC-1 511 by the quartuple (x₁,y₁, z₁, δ₁), where the location is expressed in Cartesian coordinateswith respect to a suitable reference frame.

The GPS receiver REC-1 511 continually estimates the pseudorange to allvisible GPS satellites. For explanatory purposes assume that thesepseudorange estimates are made on a time grid {T_(n)}, where T_(n) isthe GPS time associated with the n^(th) pseudorange measurementinterval.

At time T_(n), there may be K satellites visible that are indexed forconvenience as j₁, j₂, . . . , j_(K). The receiver REC-1 and the masterclock REC-M generate pseudorange estimates for the K satellites at timeT_(n). As mentioned before, it is possible to eliminate pseudorangemeasurements that are corrupted by multipath. For sake of example,assume that just one satellite pseudorange associated with satellite j₁is retained. In GPS methodology, the orbital information available forthe satellite vehicles provides a position (x_(j1), y_(j1), z_(j1)) forthe satellite at time T_(n).

The following equation(s) can be developed for the pseudorange betweenthe receiver and the visible satellite(s) that are deemed to beuncorrupted by multipath (only one equation, representing j1, is shown;additional equations may be available for additional cases wheremultipath is minimal):

$\begin{matrix}{{{\left( \frac{1}{c} \right)\sqrt{\left( {x_{1} - {x_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {y_{1} - {y_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {z_{1} - {z_{j\; 1}\left( T_{n} \right)}} \right)^{2}}} - \delta_{1}} = {{{PR}_{1\; j\; 1}\left( T_{n} \right)} + {ɛ_{1\; j\; 1}\left( T_{n} \right)}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

In (Eq. 9) the term “c” represents the speed of light, δ₁ is the timeoffset of the clock 511 (REC-1), PR_(1j1)(T_(n)) is the pseudorange (intime units) as computed in REC-1 for satellite vehicle j1, andε_(1j1)(T_(n)) represents the remaining error(s) due to measurementinaccuracies after known corrections to the range such as ionosphericcorrections have been accounted for.

Over an extended period, there may be several such equations developedfor distinct values of T_(n). Of special interest is that the samesatellite vehicle may be utilized more than once in the set of equationsdeveloped. This is permissible because the receivers are syntonized andtherefore have a good frequency base so that δ₁ does not change withtime (until corrected).

There are 4 principal unknowns: (x₁, y₁, z₁, δ₁) and, consequently,there need to be at least 4 equations to establish these values. Ifthere are only four equations then it is conventional to assume thatmeasurement errors are “zero” and the solutions are therefore colored bythe measurement errors. If there are more than 4 equations available,conventional techniques will establish a solution for these fourunknowns that minimizes the impact of the measurement error. Theequations are not linear equations, but there are conventional methodsavailable to linearize the equations or to solve them using iterativemethods such as the Newton-Raphson technique.

Having several equations, with satellite positions that are diverse,permits choosing a suitable subset of the equations. Given the satellitepositions and the approximate location of the receiver REC-1, the chosenequations will provide the best Geometric Dilution of Precision (GDOP).

Iterative Approach to Improve Location Estimation

A collection of clocks and receivers may be considered as aself-organizing network. In particular, it is possible for thecollection to detect whether a particular clock/receiver has moved.

At any given time of day, say T_(n), the location of the satellites inspace can be computed. Given the (approximate) location of tworeceivers, say (x₁, y₁, z₁) and (x₂, y₂, z₂), and a particular satellitewhose location (x_(S), Y_(S), z_(S)) is known, the pseudo rangedifferential can be computed and bounded (see Eq. 4). The pseudo rangedifferential can also be measured. If the measured value of the pseudorange differential is substantively different from the computed value,then two possibilities arise. One is that the signal to one or bothreceivers has been corrupted by multipath and the second is that one, orboth, receivers has been moved. Over the course of a day it is verylikely that there will be at least some interval where the signal is notcorrupted by multipath and within this interval the determination can bemade whether the receiver was moved or is in its expected location.Establishing multipath may involve comparing pseudo range residualsbetween several receivers in the collection.

An iterative procedure can be used to continually improve thelocation/time estimates of the receivers. In one embodiment, theiterative procedure may include the steps of FIG. 11, which depicts thefollowing method 1100 for iteratively estimating the location/time ofGNSS receivers:

Step 1101 is a background operation in which the receivers are allsyntonized using PTP over the network. Although depicted as a firststep, it should be understood that PTP may run continually, maintainingsyntonization between all the clocks. PTP is also used to estimate thetime offset between the various clocks. Until asymmetry of network pathsis estimated, the time offset of a particular Slave clock relative tothe Master could be incorrect by the amount related to asymmetry.

At step 1102, a Master receiver communicates its location to Slavereceivers, which use the Master's location as initial estimates of theirown locations. One of the receivers is designated as the Master. This isgenerally the receiver that has the clearest view of the sky. Inwireless deployments this would be the macro-cell in a collection ofmacro- and small cells. The location of the receiver is establishedusing conventional GPS/GNSS methods and used as a first approximation oftheir own location. In many cases this approximation may be adequate.PTP provides a time synchronization between the receivers that isreasonably accurate with the principal source of error being theasymmetry.

At step 1103, equations, discussed above, are established based onpseudoranges between the Slave receivers and satellites estimated atdistinct times or estimated by a plurality of the Slave receivers, andthe equations are solved to estimate the time and location at each Slavereceiver. Essentially this provides for better estimates of the time andlocation for the receiver that improve upon the initial estimate of step1102.

At step 1104, each Slave receiver may output a signal reflective of thetime and/or location estimated at that receiver. In turn, such time andlocation estimates may be used to support enhanced transmission formatsand techniques which require tight synchronization between receivers intime/phase and frequency, emergency services such as the E-911 system,among other things.

After step 1104, the method 1100 returns to step 1103, where additionalequations are established and used to solve for the time and location atthe Slave receivers.

While the forgoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof. For example, aspects of thepresent invention may be implemented in hardware or software or in acombination of hardware and software. One embodiment of the inventionmay be implemented as a program product for use with a computer system.The program(s) of the program product define functions of theembodiments (including the methods described herein) and can becontained on a variety of computer-readable storage media. Illustrativecomputer-readable storage media include, but are not limited to: (i)non-writable storage media (e.g., read-only memory devices within acomputer such as CD-ROM disks readable by a CD-ROM drive, flash memory,ROM chips or any type of solid-state non-volatile semiconductor memory)on which information is permanently stored; and (ii) writable storagemedia (e.g., floppy disks within a diskette drive or hard-disk drive orany type of solid-state random-access semiconductor memory) on whichalterable information is stored. Such computer-readable storage media,when carrying computer-readable instructions that direct the functionsof the present invention, are embodiments of the present invention.

1. A method of determining time offset and/or location of a firstreceiver, comprising: estimating a plurality of pseudoranges based onsatellite signal time delays, wherein the pseudoranges include at leastone of (a) pseudoranges between the first receiver and one or moresatellites estimated by the first receiver at distinct times, and (b)pseudoranges between one or more second receivers and one or moresatellites estimated by the one or more second receivers, the one ormore second receivers being syntonized with each other and with thefirst receiver; determining at least one of the time offset and thelocation of the first receiver by solving a set of equations formulatedbased on the estimated pseudoranges; and outputting a signal reflectiveof the determined one of the time offset and the location of the firstreceiver.
 2. The method of claim 1, wherein each of the equations in theset of equations has form${{{\left( \frac{1}{c} \right)\sqrt{\left( {x_{1} - {x_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {y_{1} - {y_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {z_{1} - {z_{j\; 1}\left( T_{n} \right)}} \right)^{2}}} - \delta_{1}} = {{{PR}_{1\; j\; 1}\left( T_{n} \right)} + {ɛ_{1\; j\; 1}\left( T_{n} \right)}}},$with c representing speed of light, δ₁ representing a time offset of aclock in the first receiver or in one of the second receivers,PR_(1j1)(T_(n)) representing a pseudorange between the first receiver orthe one of the second receivers and satellite j1, and ε_(1j1)(T_(n))representing error(s).
 3. The method of claim 1, further comprising:determining a difference between an estimated pseudorange between thefirst receiver and one of the satellites and an estimated pseudorangebetween one of the second receivers and the one of the satellites; anddetermining multipath has occurred or the first receiver or the one ofthe second receivers has moved, when the difference is greater than apredefined threshold value, wherein each of the equations in the set ofequations is formulated using a respective pseudorange estimated whenmultipath does not occur.
 4. The method of claim 3, wherein the firstreceiver or the one of the second receivers is determined to have movedif the difference between the estimated pseudorange between the firstreceiver and the one of the satellites and the estimated pseudorangebetween the one of the second receivers and the one of the satellites isgreater than the predefined threshold value for an entire day.
 5. Themethod of claim 1, wherein the one or more second receivers and thefirst receiver are syntonized using Precision Timing Protocol (PTP)streams.
 6. The method of claim 1, further comprising: receiving anestimated location of a master receiver, wherein the estimated locationof the master receiver is used as an initial estimate of the location ofthe first receiver; and iteratively performing the step of estimatingthe plurality of pseudoranges and solving sets of equations formulatedbased on the estimated pseudoranges to improve the estimate of thelocation of the first receiver.
 7. The method of claim 5, wherein themaster receiver is a macro-cell receiver.
 8. The method of claim 1,wherein a plurality of the pseudoranges are estimated based on timedelays of signals from a single satellite.
 9. The method of claim 1,further comprising: selecting a set of four equations, formulated basedon the estimated pseudoranges, so as to minimize measurement error,wherein the selected set of equations is solved to determine the atleast one of the time offset and the location of the first receiver. 10.A global navigation satellite systems (GNSS) receiver device,comprising: a memory; and a processing unit programmed to determine timeoffset and/or location of the GNSS receiver by performing operationscomprising: estimating a plurality of pseudoranges based on satellitesignal time delays, wherein the pseudoranges include at least one of (a)pseudoranges between the GNSS receiver and one or more satellitesestimated by the GNSS receiver at distinct times, and (b) pseudorangesbetween one or more other receivers and one or more satellites estimatedby the one or more other receivers, the one or more other receiversbeing syntonized with each other and with the GNSS receiver, determiningat least one of the time offset and the location of the first receiverby solving a set of equations formulated based on the estimatedpseudoranges, and outputting a signal reflective of the determined oneof the time offset and the location of the first receiver.
 11. The GNSSreceiver of claim 10, wherein each of the equations in the set ofequations has form${{{\left( \frac{1}{c} \right)\sqrt{\left( {x_{1} - {x_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {y_{1} - {y_{j\; 1}\left( T_{n} \right)}} \right)^{2} + \left( {z_{1} - {z_{j\; 1}\left( T_{n} \right)}} \right)^{2}}} - \delta_{1}} = {{{PR}_{1\; j\; 1}\left( T_{n} \right)} + {ɛ_{1\; j\; 1}\left( T_{n} \right)}}},$with c representing speed of light, δ₁ representing a time offset of aclock in the GNSS receiver or in one of the other receivers,PR_(1j1)(T_(n)) representing a pseudorange between the GNSS receiver orthe one of the other receivers and satellite j1, and ε_(1j1)(T_(n))representing error(s).
 12. The GNSS receiver of claim 10, the operationsfurther comprising: determining a difference between an estimatedpseudorange between the GNSS receiver and one of the satellites and anestimated pseudorange between one of the other receivers and the one ofthe satellites; and determining multipath has occurred, or the GNSSreceiver or the one of the other receivers has moved, when thedifference is greater than a predefined threshold value, wherein each ofthe equations in the set of equations is formulated using a respectivepseudorange estimated when multipath does not occur.
 13. The GNSSreceiver of claim 12, wherein the GNSS receiver or the one of the otherreceivers is determined to have moved if the difference between theestimated pseudorange between the GNSS receiver and the one of thesatellites and the estimated pseudorange between the one of the otherreceivers and the one of the satellites is greater than the predefinedthreshold value for an entire day.
 14. The GNSS receiver of claim 10,wherein the one or more other receivers and the GNSS receiver aresyntonized using Precision Timing Protocol (PTP) streams.
 15. The GNSSreceiver of claim 10, the operations further comprising: receiving anestimated location of a master receiver, wherein the estimated locationof the master receiver is used as an initial estimate of the location ofthe GNSS receiver; and iteratively performing the step of estimating theplurality of pseudoranges and solving sets of equations formulated basedon the estimated pseudoranges to improve the estimate of the location ofthe GNSS receiver.
 16. The GNSS receiver of claim 15, wherein the masterreceiver is a macro-cell receiver.
 17. The GNSS receiver of claim 10,wherein a plurality of the pseudoranges are estimated based on timedelays of signals from a single satellite.
 18. The GNSS receiver ofclaim 10, the operations further comprising: selecting a set of fourequations, formulated based on the estimated pseudoranges, so as tominimize measurement error, wherein the selected set of equations issolved to determine the at least one of the time offset and the locationof the GNSS receiver.
 19. A method for detecting whether a satellitesignal received by a first receiver is corrupted by multipath,comprising: estimating a pseudorange between the first receiver and asatellite based on a time delay of the satellite signal received by thefirst receiver; receiving, from a second receiver, an estimate of apseudorange between the second receiver and the satellite; determining adifference between the pseudorange estimates between the first receiverand the satellite and between the second receiver and the satellite;determining the signal received by the first receiver is a multipathsignal if the difference between the pseudorange estimates is greaterthan a predefined threshold value; and outputting a signal reflective ofthe determination of whether the signal received by the first receiveris a multipath signal.
 20. The method of claim 19, wherein the first andthe second receivers are syntonized using a Precision Timing Protocol(PTP) stream which delivers timing references from the first receiver tothe second receiver.